In general, hybrid electric vehicles (HEVs) are mounted with a battery pack in which several tens of battery cells are connected in series. The maximum available charge and discharge powers of the battery pack are controlled on the basis of a state of charge (SOC) and temperature of the battery pack. An example of determining the maximum available charge and discharge powers of the battery pack will be described below on the basis of conventional charge and discharge control using characteristic modeling of the battery constituted of charge and discharge internal resistances.
First, since the internal resistances of the battery pack are changed depending on the SOC and temperature of the battery pack, they are measured at each temperature for each SOC through a test. In a method of measuring the internal resistances, when current flows through the battery pack, a value dividing a variation of voltage by the current is determined as a value of internal resistance. The obtained internal resistance value is stored in a memory. Real power of the battery pack is estimated through the internal resistance according to Equation (1).
                              P          =                                    I              ×              Δ              ⁢                                                          ⁢              V                        =                                                                                Δ                    ⁢                                                                                  ⁢                    V                                    R                                ×                Δ                ⁢                                                                  ⁢                V                            =                                                Δ                  ⁢                                                                          ⁢                                      V                    2                                                  R                                                    ⁢                                  ⁢                              Δ            ⁢                                                  ⁢            V                    =                                    V              LIMIT                        -                          V              CURRENT                                                          (        1        )            
In Equation (1), the negative value becomes discharge power, but the positive value becomes charge power. Further, the internal resistance R is to call up the internal resistance value stored in the memory according to the SOC and temperature of each state, and then substitute the internal resistance value into the Equation (1).
Meanwhile, each battery cell has the same performance when initially mounted to a vehicle, and thus the power of a motor for the vehicle is controlled on the basis of the maximum power of the battery pack.
However, as the mileage of the vehicle increases, performance deviation is generated between the battery cells. Nevertheless, when charging and discharging processes continue to be performed on the basis of the initial maximum power of the battery pack, some battery cells are charged or discharged in excess of available charge or discharge power, as shown in FIG. 1.
In this manner, because the power of the vehicular motor is continuously controlled on the basis of the initial maximum power of the battery pack, some degraded battery cells are over-charged or over-discharged during operation of the vehicle. This results in the acceleration of the performance deviation between the battery cells, thereby sharply reducing a lifetime of the battery pack.